The problem presumably is the following: the way you're connecting STDOUT to the pipe is changing the file descriptor number so that it's no longer 1. However, file descriptor 1 is what your exec'ed child process is assuming to be stdout (i.e. the default).

printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT);

open $old_stdout, ">&STDOUT" or die "open: $!";
close( STDOUT );

pipe( $smash_stdout, STDOUT );

printf STDERR "after:  fileno(STDOUT)=%d\n", fileno(STDOUT);
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prints

before: fileno(STDOUT)=1
after:  fileno(STDOUT)=6
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Note that fileno changed from 1 to 6.  In other words, you now have fileno 6 connected to the pipe (something an exec'ed child would know nothing about...)

Something like this might work better

printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT);

open $old_stdout, ">&STDOUT" or die "open: $!";
# _don't_ explicitly close STDOUT here

local(*RH, *WH);
pipe RH, WH;
open STDOUT, ">&WH" or die "open: $!";

printf STDERR "after:  fileno(STDOUT)=%d\n", fileno(STDOUT);
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because this way, STDOUT keeps being associated with file descriptor 1:

before: fileno(STDOUT)=1
after:  fileno(STDOUT)=1
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Update: a simple demo:

open $old_stdout, ">&STDOUT" or die "open: $!";
local(*RH, *WH);
pipe RH, WH;
open STDOUT, ">&WH" or die "open: $!";

# write something to the pipe
print "foo";
# and have some fork/exec'ed process write to the pipe (via stdout)
system('echo bar');

close WH;
my $out = <RH>;

print STDERR "got from pipe: $out";   # $out is "foobar\n"
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